Olivine Fabric Research Articles

A microstructural imprint of melt impregnation in slow spreading lithosphere: Olivine‐rich troctolites from the Atlantis Massif, Mid‐Atlantic Ridge, 30°N, IODP Hole U1309D

The 1415 m deep IODP Hole U1309D (Mid‐Atlantic Ridge, 30°N) is the second deepest hole drilled into slow spreading oceanic lithosphere. The recovered section comprises essentially gabbroic rocks, with a large range of compositions. The most primitive end‐members of the gabbroic sequence, herein referred to as olivine‐rich troctolites (ol > ∼70%), have textures and geochemical compositions intermediate between that of mantle peridotites and primitive cumulates, indicative of melt impregnation processes. We carried out a detailed microstructural study to further characterize the petrogenetic processes leading to their formation, as well as discuss their mode of emplacement and relationship with neighboring mantle lithosphere. In olivine‐rich troctolites, olivines range from coarse‐grained subhedral crystals, commonly containing well‐developed subgrains, to medium‐grained rounded crystals with fewer or no substructures. They are embedded in large, undeformed pyroxene and plagioclase poikiloblasts. Olivine substructures reveal dislocation creep that is consistent with activation of the main high‐temperature slip systems, dominantly (010)[100]. Olivine crystallographic preferred orientation is very weak but generally shows a relatively stronger, uncommon [001] concentration. These unusual olivine fabrics are interpreted as resulting from melt impregnation of a previously deformed olivine matrix: the solid olivine framework is disrupted by olivine corrosion along grain and subgrain boundaries, and the high‐temperature plastic fabric is modified in a liquid‐dominated regime. Based on mineral composition and fabrics and in comparison with what is observed in impregnated mantle rocks elsewhere, we posit that olivine represents relicts of mantle peridotites disaggregated by large melt influx, although the mantle origin of olivine is not unequivocally demonstrated yet. Whatever the initial lithology, impregnation by large volumes of melt has strongly modified the original composition and microstructure. If the mantle origin hypothesis is correct, the original olivine fabric could have been efficiently weakened by dunitization prior to disruption of the olivine framework by melt impregnation. Incorporation, at the base of the lithosphere, of small slivers of impregnated dunite into gabbroic sections, trapped between successive igneous units, may be a common mechanism of lower crustal accretion at slow spreading ridges. Extensive melt‐rock interaction processes are expected to contribute significantly to the final chemical composition of erupted lavas.

Links between the structure of the mantle lithosphere and morphology of the Cheb Basin (Eger Rift, central Europe)

The Cheb Basin (CHB), located in the western part of the Eger Rift (ER) and the western Bohemian Massif, is characterized by earthquake swarms, neotectonic crust movements and emanations of CO2 dominated gases of mantle origin. Deep structure of the region can be characterized as junction of three domains of mantle lithosphere with different olivine fabrics revealed by consistent orientations of seismic anisotropy. The domains represent mantle components of the major tectonic units (micro-plates): Saxothuringian (ST), Tepla-Barrandian (TB) and Moldanubian (MD), which were assembled during the Variscan orogeny. The ST-TB boundary, reactivated during the Cenozoic extension, controlled the position and development of the ER and the CHB. We show that the CHB originated above the rejuvenated mantle suture between the ST and TB. Though the basin is located within the ST crust domain, which is thrust over the mantle junction, it is the mantle suture that controls the CHB shape and its development through the allochthonous ST crust. The seismically active Marianske Lazně Fault limits the basin against the uplifted block of the Erzgebirge Crystalline Complex. The most subsided parts of the ER and CHB developed above the centre of the mantle transition, whereas a well expressed morphology developed above its flanks. Our study documents a long memory of the mantle lithosphere assembly inherited from the Variscan orogeny. It is possible that other continental regions also contain some of intra-plate basins that originated above healed palaeo-plate mantle boundaries.

Long memory of mantle lithosphere fabric — European LAB constrained from seismic anisotropy

We present a uniform updated model of the European lithosphere–asthenosphere boundary (LAB) recalculated from data collected during our regional studies of seismic anisotropy and other tomographic experiments, and show results of mapping of large-scale domains of mantle lithosphere characterized by uniform fossil fabrics. Exploiting the long memory of the fabric of the deep continental lithosphere, we define the LAB as a boundary between a fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy related to present-day flow in the asthenosphere. Analysis of static terms of teleseismic P-wave travel time deviations shows that the LAB topography is more distinct beneath the Phanerozoic part of Europe than beneath its Precambrian part and deepens down to ∼ 220 km beneath the two Alpine roots, the South Carpathians and eastward of the Trans-European Suture Zone. Separation of the relative P residuals into static and directional-dependent terms, shear-wave splitting analysis and joint inversion of anisotropic parameters, reveals that the mantle lithosphere consists of domains with consistent olivine fabrics, which can be modelled by peridotite aggregates with plunging foliation or lineation. Changes of the fossil fabric allow us to map the domain boundaries within the mantle lithosphere. We propose processes which could create the observed fossil fabric as a consequence of successive subduction and accretion of micro-continent fragments outboard of continental cratons and a gradual stabilization of the LAB by a mantle flow after a detachment of the lower parts of subducting slabs. We advocate the importance of combining different methods and data in Earth structure studies and applying 3D approaches considering both seismic anisotropy and the general orientation of symmetry axes to construct more realistic models of large-scale lithospheric structure.

Two Contrasting Fabric Patterns of Olivine Observed in Garnet and Spinel Peridotite from a Mantle-derived Ultramafic Mass Enclosed in Felsic Granulite, the Moldanubian Zone, Czech Republic

The Mohelno peridotite is a medium-sized ultramafic body (2 km × 4 km in size) enclosed in the Gföhl granulites in the eastern part of the Bohemian Massif. It consists mainly of coarse spinel peridotite (harzburgite and dunite); garnet peridotite occurs only in the sheared and deformed margins of the body. To decipher the origin and history of this mantle-derived peridotite, we determined the mineral chemistry by electron microprobe analysis and olivine fabric patterns by the electron backscattered diffraction method for each rock type. We found two distinct types of olivine fabric (crystal-preferred orientation; CPO) in the peridotite, which can be correlated with the mineralogy and thermal history of each. The olivine CPO in coarse-grained spinel peridotite shows a strong concentration of [100] slightly oblique to the lineation and [010] and [001] girdles normal to the lineation (which is the so-called {0kl}[100] pattern typical of medium-temperature deformation). Olivine in coarse-grained garnet peridotite, on the other hand, shows a strong concentration of [010] normal to the foliation and a concentration of [100] parallel to the lineation (which is the so-called (010)[100] pattern typical of high-temperature deformation). These fabric patterns become diffuse as the grain size is reduced for each mineralogical type. We interpret the development of these contrasting fabric patterns and mineralogical types based on the pressure–temperature history of each rock type determined by applying published geothermometers and geobarometers to the constituent minerals. Starting from a high-temperature (>1200°C) spinel peridotite, during exhumation and cooling in contact with surrounding granulites, the marginal part of the body was transformed to garnet peridotite, whereas the interior remained in the spinel-peridotite facies because cooling was slower inside the body. Because of the slow cooling and continuous deformation in the interior of the body, the original high-temperature fabric pattern in the spinel peridotite was converted to a lower-temperature type. The high-temperature fabric was preserved only at the margin of the body where cooling was more rapid. Reduction of grain size that occurred during later, low-temperature, deformation partly obliterated the high-temperature fabric patterns for both garnet and spinel peridotites. The initial rapid cooling at high temperatures associated with deformation probably occurred after the mantle peridotite was emplaced within the crustal granulites, which implies that the spinel- to garnet-peridotite transformation took place in the continental crust.

Microstructural and Rheological Evolution of a Mantle Shear Zone

We conducted a microstructural study of a high-strain mantle shear zone from the Josephine Peridotite, SW Oregon, USA. The goal of this study is to understand how microstructural evolution at large strains leads to transitions in rheological behavior. The shear zone we investigated exhibits higher strain and greater localization than previously studied shear zones in the Josephine Peridotite. The margins of the shear zone have a homogeneous microstructure, characterized by moderately strong olivine fabrics, fairly weak orthopyroxene fabrics, and grain sizes of 2^3 mm. The highly deformed samples from the center of the shear zone display two distinct microstructural domainsca relatively coarse-grained domain (� 550m) that contains only olivine and a finer-grained domain (� 250m) that contains both olivine and orthopyroxene. The coarse-grained domain has a strong E-type olivine lattice-preferred orientation (LPO). Within the fine-grained domain the olivine LPO is also E-type, but significantly weaker.The E-type fabrics are rotated slightly past the shear plane, providing the first field-based confirmation of similar experimental observations. The presence of E-type fabrics, which form in the presence of moderate quantities of water, also highlights the potential importance of water to shear zone evolution.The orthopyroxene in the fine-grained domains has no LPO, suggesting that a transition to grain-size sensitive deformation occurred. The microstructural transition in orthopyroxene may have resulted in a marked weakening of the rock, suggesting that orthopyroxene plays a critical role in shear localization. These samples provide a crucial microstructural link between moderately localized shear zones and highly deformed ultramylonites.

Serpentine Mineral Replacements of Natural Olivine and their Seismic Implications: Oceanic Lizardite versus Subduction-Related Antigorite

We report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations. In the first case, mesh-textured lizardite (liz) is developed in a standard 60% serpentinized oceanic harzburgite from the Oman ophiolite where olivine converts to columnar lizardite. The joined columns are perpendicular to the basal plane (001)liz, corresponding to the pseudofibres observed optically. The plane (001)liz is locally parallel to the narrow boundary ol–liz; thus column orientations register the interface of serpentinization. The ol–liz relationships are not strictly topotactic, but reflect preferred cracking orientations in olivine, parallel to (010)ol. In the second case, antigorite (atg) develops in a rare sample of antigorite schist in a kimberlite from Moses Rock (Colorado Plateau), representative of a suprasubduction-zone mantle wedge. High-resolution transmission electron microscopy (HRTEM) images along [010]atg show domains of very regular modulation with a 43·5 Å wavelength (m = 17, where m is the number of silicate tetrahedra along the wave), with few defects, indicative of HP–HT antigorite, and also heavily kinked regions as fingerprints of strong tectonic shear. TEM imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine: [100]atg//[010]ol and <100>atg//<100>ol; the planes in contact are (001)atg//(100)ol and (001)atg//(010)ol, respectively. The [010]atg//[001]ol and antigorite lamellae are parallel to the forsterite b-axis. In both cases, the topography of olivine–serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate. In the cases studied, the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite. These results have some relevance to the seismic anisotropy of serpentinized mantle. Anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite. Conversely, the development of antigorite produces a trench-parallel fast S-wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization.

Shear-wave splitting, lithospheric anisotropy, and mantle deformation beneath the Arabia–Eurasia collision zone in Iran

Records of core-refracted phases from permanent and temporary seismological stations are analysed to provide a map of seismic anisotropy covering most tectonic provinces of Iran. Anisotropy is coherent at the scale of a single province, but it differs from one province to another. No significant shear-wave splitting is observed in the mountain ranges of Zagros, Alborz, and Kopeh-Dagh. Anisotropy with average time lags of 1.1 ± 0.3 s between fast and slow shear-waves with axes of the faster polarization oriented mostly NW–SE is observed in Central Iran, but the faster orientation is NE–SW in NE Iran. The orientations of fast axes do not coincide with the directions of absolute plate motion in the no-net rotation frame or in the hot-spot frame, suggesting that the observed anisotropy cannot be explained by a simple model of asthenospheric flow. The heterogeneous orientation of fast axes with differences in length scales of a few hundreds km suggests that the anisotropy originates in the lithospheric mantle. In the area where strike-slip shear dominates, the fast axes lie approximately parallel to the orientations of maximum shear estimated from GPS measurements, either with right-lateral shear associated with the NS-trending right-lateral faults in Central Iran, or left-lateral shear associated with EW-trending left-lateral faults in Northeast Iran. In the mountain belts, the null measurements may be related to horizontal shortening and thickening by pure shear, as proposed from synthetic models of olivine fabric which both imply a large amount of vertical strain, not seen by SKS splitting. Our data therefore do not agree with previous explanations of shear-wave splitting in the neighbouring regions of Eastern Anatolia and Saudi Arabia in terms of simple asthenospheric flow. They rather support the view that Iran deforms over a broad area, with vertically-coherent deformation at a lithospheric scale, that includes both shear between blocks in Central Iran and shortening at the mountain belts.

Rock seismic anisotropy of the low‐velocity zone beneath the volcanic front in the mantle wedge

Peridotite xenoliths derived from the low velocity zone beneath the Avacha frontal volcano, Kamchatka, preserve a‐axis slip fabrics, comparable with those in xenoliths from the back‐arc region of the NE Japan. Although low‐velocity zones are commonly attributed to zones of partially melted mantle, migration of the melt does not erase the existing olivine fabrics and related seismic anisotropies. These anisotropies may counteract the anisotropies associated with c‐axis slip fabrics, if they exist, along the slab or in the high‐pressure zone.

Bottom to top lithosphere structure and evolution of western Eger Rift (Central Europe)

We present model of the structure and development of the entire lithosphere beneath the western Eger Rift (ER). Its crustal architecture and paths of volcanic products are closely related to sutures/boundaries of uppermost mantle domains distinguished by different orientations of olivine fabric, derived from 3-D analysis of seismic anisotropy. Three different fabrics of the mantle lithosphere belong to the Saxothuringian (ST), Tepla-Barrandian (TB) and Moldanubian (MD) microplates assembled during the Variscan orogeny. Dipping fossil (pre-assembly) olivine orientations, consistent within each unit, do not support any voluminous mantle delamination. The variable rift structure and morphology depend on the character of the pre-rift suture between the northern ST unit and the TB/MD units in the southern rift flank. The proper rift with typical graben morphology has developed above the steep lithosphere-scale suture between the ST and TB units. This subduction-related boundary originated from the closure of the ST Ocean. Parts of the crust and mantle lithosphere were dragged there into asthenospheric depths and then rapidly uplifted. The suture is marked by abrupt change in the mantle fabric and sharp gradients in regional gravity field and in metamorphic grade. The secular TB-side-down normal movement is reflected in deep sedimentary basins, which developed since the Carboniferous to Cenozoic and in topography. The graben morphology of the ER terminates above the “triple junction” of the ST, TB and MD mantle lithospheres. The junction is characterized by offsets of surface boundaries of the tectonic units from their mantle counterparts indicating a detachment of the rigid upper crust from the mantle lithosphere. The southwest continuation of the rift features in Bavaria is expressed in occurrences of Cenozoic sediments and volcanics above an inclined broad transition zone between the ST and MD lithospheres. Schematic scenario of evolution of the region consists mainly of a subduction of the ST lithosphere to depths around 140 km, exhumation of HP-HT rocks and the post-tectonic granitoid plutonism.

Insights into the nature of plume–asthenosphere interaction from central Pacific geophysical anomalies

The central Pacific is one of the type localities where the influence of a plume on the properties of the asthenosphere can be investigated based on the geophysical observations. Recent seismological studies showed enigmatic structures of the upper mantle in the central Pacific. The asthenosphere (~ 100–200 km depth) in this region has unusually strong V SH/ V SV > 1 anisotropy associated with relatively weak azimuthal anisotropy. Although less well-constrained, there are suggestions of higher than normal viscosity and lower than normal electrical conductivity in this region. Previous models for the plume–asthenosphere interaction do not easily explain these observations. I propose that these observations are the consequence of a single process: depletion of water by deep partial melting in the plume column. The plume-fed materials in the asthenosphere will be dominated by the olivine A-type fabric whereas in the surrounding normal asthenosphere olivine E-type fabric dominates. This contrast in olivine fabrics provides an explanation for the observed anomalous seismic anisotropy in the central Pacific. Water depletion will also cause high viscosity and low electrical conductivity. The present model implies that plumes supply depleted materials (rather than enriched materials) to the asthenosphere.

Seismic anisotropy — A key to resolve fabrics of mantle lithosphere of Fennoscandia

By merging different regional sets of body-wave data, we compile a relief of the lithosphere–asthenosphere boundary in one of the oldest continental regions on the planet: the Fennoscandian Shield. A maximum lithospheric thickness of 200–220 km is observed in the central part of the Shield and a significant thinning at its margins, particularly in the southwest at the Trans-European Suture Zone. Fabric of the mantle lithosphere is modelled from independent sets of anisotropic parameters, relative P-residual spheres and shear-wave splitting. The mantle lithosphere of Fennoscandia consists of several domains consistent olivine fabrics of dipping foliations, in 3D anisotropic models of the Proterozoic lithosphere, or, predominantly dipping lineations, in models of the Archean lithosphere. The strength of seismic anisotropy agrees with findings from mantle xenoliths. We interpret the anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric, which was created before these micro-continents assembled. We envisage an early form of plate tectonic processes, such as successive subduction episodes, stacking of oceanic plates, and accretion of magmatic arcs. Such processes could create the generally observed fossil dipping fabrics of the thick Precambrian mantle lithosphere, starting as early as the Archean.

Mantle fabric of western Bohemian Massif (central Europe) constrained by 3D seismic P and S anisotropy

New teleseismic data from a dense network of temporary and permanent seismic stations were analyzed to investigate large-scale fabric of the lithosphere–asthenosphere system beneath the western part of the Variscan Bohemian Massif. We study three-dimensional orientation and strength of seismic anisotropy and model a fabric of the mantle lithosphere from shear-wave splitting and directional terms of relative P residuals. Fast shear-wave polarizations have mostly E–W orientation and split-delay times δt are around 1.2 s, on average. Small but systematic changes of the fast S split polarizations show a difference at about 20° to 30° between the Teplá-Barrandian (TBU)/Moldanubian (MD) tectonic units and the adjacent part of the Saxothuringian (ST), and about 40° between two parts of the ST, separated by a suture identified in the upper crust by different authors and by other methods. On the other hand, P-velocity anisotropy clearly indicates three different orientations of fossil olivine fabrics with inclined axes of symmetry in the mantle lithospheres of three tectonic units (ST, TBU, MD). The seeming contradiction between the results of the two independent data sets can be explained by different 3D anisotropic models: one with hexagonal ‘slow’ symmetry axis b and divergently dipping ( a, c) foliations (ST, MD) and the other with hexagonal or orthorhombic symmetry with dipping high-velocity lineation a (TBU). These 3D self-consistent anisotropic models are compatible for both the P- and S-anisotropy observables. Our results suggest that a directionally varying constituent of the anisotropic signal is “frozen” in the mantle lithosphere. Regional changes in seismic anisotropy thus map tectonic boundaries, which cut the whole lithosphere, though with shifted crustal and mantle parts (TBU). Depending on symmetry and orientation of fabrics of the lithosphere domains, lateral changes are reflected in either P-velocity anisotropy and/or shear-wave polarizations. About half of the shear-wave split-time delays with predominantly E–W polarization azimuths represent a constant anisotropic signal which we associate with an olivine preferred orientation due to present-day flow in the asthenosphere. Our study emphasizes the importance of combining different methods of analysis and using complementary data sets in the three-dimensional analysis of mantle fabrics.

Deformation fabrics of olivine in Val Malenco peridotite found in Italy and implications for the seismic anisotropy in the upper mantle

An exhumed huge body of ultramafic rocks is preserved in Val Malenco, Italy. In this study, petrofabrics and deformation microstructures of olivine in the Val Malenco peridotite were examined, and new observations of lattice preferred orientation (LPO) of olivine such as type-B and type-E LPO in the peridotite are reported. Three types, depending on the grain size, of olivine fabrics were observed: type-A fabric exhibiting the (010)[100] slip system in the large relic grain area, and type-B and type-E fabrics exhibiting the (010)[001] and (001)[100] slip systems, respectively, in the widely distributed recrystallized-grain area. The slips occurred mainly on (010)[001], and the (001)[100] slip system has only rarely been reported to be the dominant slip system of olivine found in natural rocks. Further, seismic velocities and seismic anisotropies were calculated for the three types of fabrics found. The seismic anisotropies in the upper mantle can be attributed to the type-B, type-E, or type-A LPOs of olivine. Moreover, the anomaly in seismic low velocity in the mantle wedge can be attributed to the serpentinization as well as the mixed fabrics of the type-A, type-B, and type-E.

B-type olivine fabrics developed in the fore-arc side of the mantle wedge along a subducting slab

B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg + Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr + Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites, as their formation requires hot, hydrous, shallow mantle (> 1250 °C at < 30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880–1030 °C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions.

Olivine fabric transitions and shear wave anisotropy in the Ryukyu subduction system

Recent shear wave splitting measurements from the fore-arc region of the Ryukyu subduction system show large magnitude (0.3–1.6 s) trench-parallel splitting in both local and teleseismic phases. The similarity of splitting parameters associated with shallow local-S and teleseismic phases suggests that the source of anisotropy is located in the fore-arc mantle. One explanation for this pattern of shear wave splitting involves a transition from commonly observed high-temperature olivine fabrics with flow-parallel seismically fast directions to a flow-normal B-type olivine fabric in the cold fore-arc mantle of the Ryukyu wedge. We test the B-type fabric hypothesis by comparing observed splitting parameters to those predicted from geodynamic models that incorporate olivine fabric development. The distribution of olivine fabric is calculated with high-resolution thermomechanical models of the Ryukyu subduction zone that include realistic slab geometry and an experimentally based wet olivine rheology. We conclude that B-type fabric can explain the magnitude and trench-parallel orientation of deep local-S phases that sample the core of the fore-arc mantle. However, our calculations show that B-type fabric alone cannot account for large magnitude trench-parallel splitting associated with teleseismic phases that sample the shallow tip of the fore-arc mantle. Alternative models for trench-parallel teleseismic splitting in the shallow tip of the fore-arc mantle involve the addition of crustal or slab anisotropy and highly anisotropic foliated antigorite serpentinite.

Variable microstructure of peridotite samples from the southern Mariana Trench: Evidence of a complex tectonic evolution

We retrieved samples of peridotite from a dredge haul (KH92-1-D2) collected during Cruise KH92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11°41.16′N, 143°29.62′E; depth 6594–7431 m), which is the deepest ocean in the world. Ten of 30 retrieved samples possessed both a foliation and lineation, as assessed from 46 thin sections of various orientations and observations of hand samples. The samples showed marked variation in microstructure, ranging from coarse (> 5 mm) equigranular and intensely elongated textures to finer (< 1 mm) porphyroclastic and fine-grained equigranular textures. Olivine fabrics also varied among the different samples, with (010)[100] and (010)[001] patterns (termed A- and B-type, respectively) observed in samples with coarse textures and no clear patterns observed in samples with fine textures. Even though the peridotite samples were retrieved from a single dredge site, some contain primary tectonic microstructures and some contain secondary microstructures. Recent bathymetric and topographic analyses indicate that the lithosphere in this region is as thin as 20 km. Such a thin lithosphere may have been intensely deformed, even perhaps in the ductile regime, during fore-arc extension; consequently, the observed variations in microstructure within the peridotite samples probably reflect the complex tectonic evolution of the southern Mariana region.

Stress, strain, and B‐type olivine fabric in the fore‐arc mantle: Sensitivity tests using high‐resolution steady‐state subduction zone models

Shear‐wave splitting observations at several subduction systems show trench‐parallel fast directions in the fore‐arc mantle. The presence of B‐type olivine fabric in the mantle wedge may provide an explanation for this pattern of anisotropy under low‐temperature and hydrated conditions. Sensitivity tests are shown that provide insights into the distribution and magnitude of B‐type fabric using two‐dimensional, high‐resolution, kinematic‐dynamic subduction zone models. These models include a wet olivine rheology and a simulated aseismic creep zone in the fore‐arc mantle with parameterized viscous coupling between the slab and mantle wedge. The calculated thermal structure is in reasonable agreement with heat flow observations and Q tomography for small amounts of coupling and moderate amounts of shear heating in the seismogenic and aseismic creep zones. These small amounts of viscous coupling give rise to a slowly flowing fore‐arc mantle with thermal, stress, and strain conditions suitable for significant B‐type fabric development. The minimum shear stress necessary to drive geologically significant flow in the predicted B‐type region is sensitive to the magnitude of shear heating along the aseismic creep zone and ranges from less than 100 to 300 MPa. The minimum time required to generate sufficient finite strain for fabric development in the B‐type region is around 10 Myr depending on the amount of coupling and shear heating. The predicted distribution of olivine fabric is consistent with a pattern of shear wave anisotropy with trench‐parallel fast polarization directions above the fore arc and a rapid rotation to trench‐perpendicular in the arc and back arc.

Shear wave splitting as a diagnostic of variable anisotropic structure of the upper mantle beneath central Fennoscandia

We analyze splitting of shear waves recorded during the SVEKALAPKO passive seismic experiment in south-central Finland to study fabrics of the mantle lithosphere of the Precambrian region and thus to bring information into a debate on existence of plate tectonics or its forms in the early stage of continent formation. Geographical variations of the splitting parameters and their distinct dependence on direction of wave propagation through the upper mantle allow us to identify six domains of the central Fennoscandian mantle lithosphere, including the Proterozoic–Archean transition, and to model their fabrics by joint inversion of body wave anisotropic parameters. Fabrics of the Archean mantle lithosphere can be approximated by a peridotite aggregate with lineation a dipping to the NE. On the other hand, anisotropy of the Proterozoic mantle lithosphere is weaker and we model its fabric by the (a, c) foliations dipping to the SE. We present a 3D self-consistent anisotropic model of the Proterozoic and Archean upper mantle along the SW-NE profile in the south-central Finland. Boundaries of inter-growing wedges of the Proterozoic and Archean mantle lithospheres explain the longitudinal and shear wave propagation and polarization, mantle xenolith ages, surface wave tomography and location of the upper mantle reflectors. We interpret the six anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro-continents assembled.

Effect of temperature on the B- to C-type olivine fabric transition and implication for flow pattern in subduction zones

The effect of temperature on the B- to C-type fabric transition in olivine was investigated by large-strain shear deformation experiments. A new series of deformation experiments was conducted at 1273–1373 K, 2.0 GPa and strain-rates ranging from 3.8 × 10 −4 to 3.2 × 10 −5 s −1 under water-saturated conditions, and combined with previous results at 1473 K, the influence of temperature on deformation fabrics was investigated. Two types of olivine fabrics were identified, one in which the olivine [0 0 1] axis is subparallel to the shear direction and the (0 1 0) plane is parallel to the shear plane (B-type), and another one in which the [0 0 1] axis is parallel to the shear direction and the (1 0 0) plane is parallel to the shear plane (C-type). The B-type olivine fabric occurs at higher stress than the C-type, but the stress magnitude at which the B- to C-type fabric transition occurs decreases significantly with decreasing temperature. The temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study. When the experimental results are extrapolated to lower temperatures, the fabric transition between B- and C-type is predicted to occur at ∼1 MPa and ∼1000 K. This prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate-convergent regions. The present study indicates that when some amounts of water (>200 ppm H/Si) are present, the B-type fabric will dominate in low temperature regions of the upper mantle whereas the C-type (or E-type) fabric will dominate at high temperatures. The complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables, such as temperature and water content rather than by complexities in mantle wedge flow pattern.

The effect of dynamic recrystallization on olivine fabric and seismic anisotropy: Insight from a ductile shear zone, Oman ophiolite

Subhorizontal mantle structures subparallel to the Moho are rotated into NW–SE subvertical orientations across a shear zone in a sinistral sense of shear within the northern Fizh mantle section of the Oman ophiolite. Dynamic recrystallization resulted in grain size reduction of olivine and the development of porphyroclastic texture. Mean olivine grain size stabilized at ∼0.7 mm within the shear zone center; this may reflect the steady-state grain size of dynamically recrystallized olivine, as determined by the deviatoric stress, which in this case was as low as 10 MPa. Crystal-preferred orientation (CPO) patterns of olivine are consistently [100]-fiber or partial fiber texture, indicating that olivine slip systems did not change during shearing. Dynamic recrystallization causes a weakening of olivine fabric intensity toward the shear zone center, but this weakening is counterbalanced by CPO strengthening due to dislocation glide. This process resulted in an abrupt decrease in seismic anisotropy at the center of the shear zone, in contrast to a gradual decrease in olivine fabric intensity and mean grain size. The measured seismic anisotropy patterns did not change in ways that would be significantly measurable by seismological observations. Despite the development of the shear zone, dispersion of both P- and S-waves in the shear zone may be of little effect with respect to the overall seismic anisotropy. This is not only because the shear zone occurs substantially in a narrow region but also because the seismic anisotropy is weaker in the shear zone than the high- T structure region. It suggests that a record of simple systematic seismic anisotropy observed in the upper mantle may indicate a simplified mantle flow structure, as localized structures may be obscured in the region of the observation.